In the past, patients with HER2-positive breast cancer generally had unfavorable outcome compared with HER2 negative cancers, but the prognosis of HER2-positive locally advanced or metastatic breast cancers (MBCs) has dramatically improved due to the introduction of trastuzumab, pertuzumab, and trastuzumab emtansine (T-DM1). The gene discussed is ERBB2; the disease is cancer.